What nationality are you?
What permission do you have to be in the UK?
How long have you lived in Bahrain?
Have you kept up national insurance payments?
Have you registered with a GP surgery?
You would need to prove that you have been resident in the UK and have been making national insurance contributions. If you have been abroad for a period of time, this may mean you have to top up national insurance payments but you would not have to pay for treatment.
I hope this answers your questions. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.
You will have to provide a copy of your tenancy agreement and passport to the NHS department providing the treatment. It may transpire that you may have to top up national insurance contributions.